It is known to use quadrupole mass filters so as to selectively transmit ions within a specific range of mass to charge ratios. A quadrupole mass filter transmits ions that satisfy conditions of stability within the quadrupole field, wherein the stability conditions are defined by the dimensionless parameters q and a:
                    q        =                              4            ⁢            e            ⁢                                                  ⁢            V                                              r              0              2                        ⁢                          ω              2                        ⁢            m                                              (        1        )                                a        =                              2            ⁢            e            ⁢                                                  ⁢            U                                              r              0              2                        ⁢                          ω              2                        ⁢            m                                              (        2        )            where e is the charge of the ion, V is the amplitude of the RF voltage applied to the quadrupole electrodes, r0 is the inscribed radius between the rods of the quadrupole, w is the angular frequency of the RF voltage applied to the quadrupole (in radians/sec), m is the mass of the ion, and U is the resolving DC voltage.
Ions having values of a and q that result in unstable ion trajectories generally impact on the rods of the quadrupole and are lost. This property is exploited when the quadrupole rod set is used as a mass filter, such that the majority of the ions that are not desired to be transmitted by the mass filter impact on the inner surfaces of the rod electrodes. However, over time the inner surfaces of the rods become contaminated by the ions and the electronic charge builds up on their surfaces. Eventually, local charging of the contaminated surfaces results in degradation of performance of the mass filter. This may result in loss of transmission, loss of resolution or poor peak shape. If this occurs the mass filter must be removed from the vacuum chamber and cleaned.
It is therefore desired to provide an improved mass and/or ion mobility spectrometer, an improved method of mass and/or ion mobility spectrometry, and an improved mass filter.